Google Voice's inbound telephone numbers are intended to be issued only to users located within the USA.
You can, however, make outbound calls using an OBi with a Google account. Your calls will not have an outbound caller ID to display to the called party (they'll see "Unknown" or "Private caller" instead).
Of course, there are plenty of other service providers you can use with your OBi, instead of Google Voice. Many of us here are happy with the service from Callcentric,
voip.ms, Phonepower, and others.